Method of manufacturing electron discharge devices



5, 1935. w. L. PARROTT METHOD OF MANUFACTURING ELECTRON DISCHARGE DEVICES Filed Oct. 26, 1929 INVENTOR WLPARRo T 777 7* ATTORN Patented Feb. 5, 1935 UNITED STATES PATENT OFFICE METHOD OF MANUFACTURING ELECTRON DISCHARGE DEVICES Pennsylvania Application October 26, 1929, Serial No. 402,559

1 Claim.

This invention relates to electron discharge devices and more particularly to electron discharge devices incorporating an electron emitting hot cathode of the indirectly heated type, the heater element of which is energized by an alternating electric current, and specifically relates to an improved type of heater element employed in said cathode, and to the method of manufacturing the same. This invention is an improvement in the inventions set forth in copending application Serial No. 292,116 filed July 12, 1928 and application Serial No. 372,331 filed June 20, 1929 by John W. Marden and Ernest Anton Lederer, which applications are assigned to the same assignee as the present invention.

In copending application Serial No.- 292,116

I above identified is disclosed an electron discharge device incorporating an electron emitting hot cathode of the indirectly heated type, in which is employed a coil type heater element energized by electric curreint. In order to prevent deleterious electrical discharges from the heater element to the cooperating electrodes of the device a dielectric shield member is interposed between the heater element lead wires and said cooperating electrodes.

In the specific embodiment disclosed the cathode is comprised of a tubular metal sleeve memher having an exterior coating of electron emitting material, refractory insulating end plug members, lead wires extending through the end plug members, and a centrally disposed coil type heater element electrically connected to said lead wires.

An electron discharge device incorporating this type cathode in accordance with the specific embodiment therein, has the cooperating electrodes concentrically arranged about the cylindrical sleeve member of the cathode.

In order to prevent deleterious overheating of the interposed control electrode which would cause undesirable electron emission from said control electrode it is customary and desirous to employ a perforated or mesh type anode, the openings in such an anode allowing of greater heat radiating or heat dissipating properties than in a solid electrode. In such an electrode structure one lead wire of the heater element in the cathode must extend a portion of its length adj acent the exterior surface of the anode in order to be electrically connected to the upper end of the coil type heater element. During operation of the device it is found that electrons flowing from the cathode 'to the perforated anode are precipitatedthrough the perforations and are attracted to the lead wire carrying the alternating electric current, creating thereby deleterious hum characteristics in the plate circuit of the device employing the same. By the interposition of a dielectric shield between the lead wire and the anode this source of deleterious electric discharge is eliminated.

It has been found, however, that another source of deleterious hum is obtained from the heater element itself, as is set forth in copending application No. 372,331 above identified, and is caused by the thermionic activity of the said heater element at the temperature of operation. This source of hum is eliminated in accordance with the invention of this copending application by reducing the temperature of operation of the heater element to below the temperature of active electron emission therefrom supplying thereto at this temperature the desired wattage of electrical energy to heat by radiation the enclosing tubular metal sleeve member to a temperature at which the exterior coating of thermionic material efficiently operates.

It is found, however, that a third source of hum is caused by the deleterious electrical effects incident to the use of alternating electric current in energizing the cathode heater element, which source is mainly what is known as an electro-static effect produced in the cathode sleeve member by electric currents induced or produced therein by the coiled heater element through which the alternating electric current flows.

This electrical effect may be substantially eliminated in the cathode of the present invention by a proper spacing of the centrally aligned heater element from the tubular metal sleeve member which may be obtained by increasing the internal diameter of the said sleeve member. The rule appears to be that the closer the spacing, or coupling as it is more commonly spoken of, the greater this electro-static effect, the greater the spacing the less the hum.

In radio tube manufacture the specific spaced relationship of the incorporated elements or electrodes, relative sizes, and the like dimensional measurements pronouncedly affect the electrical characteristics of the device. In present day electrical circuits, such as radio circuits. employing such devices, certain electrical characteristics as are likely to be affected by such spacing and size of incorporated electrodes are rigidly specified and adhere to. This makes it necessary to limit the spacing of heater element and cathode sleeve member in order to keep within the desired electrical characteristic limits and as a consequence become apparent as the invention is more fully It is another object of this invention to provide".

an indirectly heated cathode having a heater element energized by alternatingelectric-current, in which the diameter of the sleeve member and the specific spaced relationship of heater element and sleeve member may be widely varied without deleteriously introducing undesirable electrical disturbances. r 7

Another object of this invention is to provide an electron discharge device'incorporatingan indirectly heated cathode having' a heater element energized by alternating electric current and at least one cooperating electrode, in which the.

specific spaced relationship of'said cathode and heater element and said cooperating electrode maybe widely varied without the introduction of deleterious electrical disturbances or effectscommonly characterized as hum during operation of the device.

' Another object of this invention is to provide a cathode assembly of the indirectly heated type, the heater element of which is energized by alternating electric current, in which inductive cou pling between said heater element and the remaining'elements of said cathode is substantially eliminated. V a These and other objects of the invention will disclosech r V In accordance with the objects of my invention I have found that I may substitute for .the usual coil type heater element heretofore employed, in

. an indirectly heated cathode, a non-inductively wound heater coil, constructedand electrically insulated inaccordance with the process hereinafter disclosed, which in combinationwith thedielectric shield feature of copendingapplication Serial No. 292,116 and the low temperature oper- .ating heater element feature vof copending' application Serial No. 372,331 above identifiedpermits the use of the mesh plate electrodes desired 7 for the purposeof cutting down the. internal heatin 'cfiects and permits greater flexibility in the spacing of the elements of said cathode and of the cooperating electrodes in an electron discharge device such as has not heretofore beenpossible'to obtain, thereby producing an electron discharge device-in which the deleterious hum character-. istics have been reduced to a substantial unmeasurable amount. a. r

Before further disclosing-the nature of my in vention reference should be had. to the .accom panying drawing setting forth the elements of the present invention,wherein- I y y g a I Fig. l 'isan enlarged side elevational View of-a non-inductively wound twin coil. helical heater element constructed in accordance-with the'present invention;

Fig. 2 is an enlarged side elevational view of an assembled cathode with the tubular-metal sleeve member cutaway, and with the-end plug members 'andthe dielectric shieldmeinbers shown incross-section;

Fig. 3 is a side elevational view partly in cross section of a radio tube mount incorporating the cathode shown in Fig. 2, with the cooperating electrodes cut away to show the spaced relationship of the elements;

Fig. 4 is a side elevational view of a 3-electrode radio tube of the type known to zthe trade as a,

227 tube incorporating the features gor, the present invention, with the enclosing glass envelope partly broken away; and a Fig. 5 is a sideeleyational view of a i-electrod radio tube of thetypeknownto the trade as a 224 tube or screen grid tube incorporating the features of the present invention with the enclosing glass envelope partially broken away and the cooperating electrodes partly in cross-section to show the spacing of the elements of the device.

The twin coil winding of the non-inductive helical heater element of the present invention,

illustrated in Fig. 1, is of the type commonly.

known in the electrical art and has heretofore been variously employed in that art where a neu tralization of -induced electrical currents in' a coil of wire energizedby alternating current is desired. Application of this non-inductive winding principle has not heretofore been made'in the heater element art, or to'the heaterelement'of indirectly heated electron emitting cathodes, or to electron discharge devices incorporating such indirectly heated cathodes. M a 1 In accordance withthe present invention I form the heater element as shownin Fig. 1 of a refractory. metal filament 1 preferably of tungsten" which is'pf a predetermined and selected size ,and length, to conform with the electrical of filamentjis'then bent-in theshape Offa hair? .andphysical characteristics desiredl "I'his length pin, the center loop portionjengagedina' slot'lin' z the end of a mandrel o f thedesired'diameteri and the two ends of the Wirewound about the mandrel in parallel spaced "relationship the desired number of turns. v v g ,The specific pitch to the turns and number of turns employed per inch depends upon the particular type cathodesleeve into which it is to be incorporated, the diameter and length of the heater element wire, and other factors. In winding the coil each set of two turns 'of they/ire are spaced equidistant' from the next setof two turns while the, two wires of each particular set are equidistant frorn each other. This relative arrangement'flof the-twin coil filament gives the I maximum non-inductive effect. I t

g In the present .instance, an indirectly heated cathode which; is applicable in the 227 type radio tubef'shown in Fig. 4, is indicated in enlarged view inFig. 2. The tubular metal cathode sleeve member 2 is approximately 120 to 1 40'mil in diameter and about 14.5 millimeters" in length.

and has an exterior thermionic electron 'emitti'ng coating '3 a of about 9.0 millimeters length.

'It is .desirable to-wind the helix of the coilin such manner that the bulkof the heatingefiect of the coil is produced within the sleeve member adjacent to this externalcoatirigB.

In the specific embodiment] shown'thef twin coil heater elementl is comprised ofapproxi mately, 6.5 mil tungsten filament, preferably of the soacalled non-sag type. At this diameterfthe length requisite to give a'current consuming'c'a- I 'pacity or resistance at the required 2.5 volts'op eration voltage is 13.5 millimeters; To properly position this length filamentwi'thin the-cathode sleeve member so that the maximum heating effect-may be obtained I prefer to form the twin is v coil upon a mandrel of about 25 mil diameter winding the twin turns spaced about 1.0 millimeter having 10 twin turns of 20 single turns, or a rating of 20 double turns per inch or 40 single turns per inch.

To insure the retaining of the turns of the twin coil in proper spaced relationship, I mount the twin coil heater element upon a core of refractory material of substantially the same approximate diameter as the mandrel upon which the coil is wound, properly positioning the coil thereon, and then subject the filament to an anneal operation such as by passing the mounted coil through a high temperature heat zone, preferably in a non-oxidizing atmosphere for the requisite time interval to effect the removal of work hardening effects. I have found that tungsten coils are thusannealed when heated to 1400 C. for a period of about l minutes.

' The annealed coil and refractory'insulating core is then removed from the furnace and the exterior surface of the wire sprayed with a coating of refractory insulating material suspended in an organic binder such as amylacetate-nitro cellulose admixture; after which the assembly of helical filament, core and coating is again baked at about 1400 C. for a few minutes to drive oil? the binder of the coating and to consolidate the refractory insulating coating to the surface of the wire. The refractory coating thus applied is about .004 inch thick, and weighs in the neighborhood ofabout milligrams per coil.

The specific composition of the refractory insulating core and sprayed coating, may be widely varied depending upon the refractory metal employed, the desired operating temperatureof the filament and the like factors. I have found that a substantially pure aluminum oxide, A1203 to which has been added a small proportion of fritting agent, such as tale, is most suitable for the purposes of the present invention.

This particular insulating material is disclosed in application by C. V. Iredell, Serial No. 308,139 filed September 24, 1928, now Patent 1,838,766, which application is assigned to the same assignee as the present invention. The core may be prepared by the extrusion method of forming refractory insulators which has heretofore been disclosed in application by J. W. Marden and F. H. Driggs, Serial No. 233,543 filed November 16, 1927 which application is also assigned to the same 'assignee as the present invention.

I may, however, employ refractory insulating material in forming the core and in coating the wire of a composition of materials such as set forth in application by F. H. Driggs, Serial No. 306,291, filed September 15, 1928, now Patent 1,826,510. The most satisfactory coating for most purposes as well as the most inexpensive and readily applied is talc, chemically identified as magnesium silicate. This compound may be readily purchased upon the market in a state of fair degree of purity and under the temperature conditions of operation of the present type heater element is sufficiently refractory for all practical purposes, and has the desirable dielectric property at that operating temperature.

The completed annealed and insulated twin coiled helical heater element thus prepared is then ready for mounting in the cathode element of an electron discharge device, as is shown in Fig. 2. The particular method of mounting the heater element in the cathode, and the manner of assembling the same with the cooperating electrodes of a radio tube'of the 227 type is specifically showninFig.3.

The usual stem tube 18is employed having at one end a flare (not shown) which is adapted to be sealed to an enclosing glass envelope and at the other end a press or seal portion through which extend leading in support wires 9, 10, 11, 12 and 13, two of which, 12 and 13 respectively, extend beyond the press a suiilcient distance to serve as supporting members for the electrode structure of the device. A refractory insulating dielectric shield member, preferably comprised of sheet mica. in round discform approximately 1%; inches in diameterhaving suitable perforations to permit support wires 12 and 13 to extend therethrough is securely mounted upon the support wires 12 and 13 by means'of welding the sleeve portion of eyelets 7, '1 to the support wires allowing the collar of the eyelet to serve as rests or supports for 'the mica disc. The cathode element including the tubular metal sleeve member 2, and plugmembers '4, 4 and centrally aligned insulated twin coil helical heater element 1, with the apex engaged by refractory metal guide member 5 extending through upper end plug member 4, is then positioned on the lower dielectric shield member 6, the filament terminals 23 and 24 extended through an opening in the shield 6 and welded=to the current conducting lead wires 9 and 10 in the manner-shown.

The cathode sleeve member 2 is then electrically connected to the lead wire 11 through an opening in the mica. disc member v6,'and the cooperating control'electrode 16 and plate electrode 1'7 suitably mounted concentric to the cathode and electrically connected tothe leading-in support wires 12 and 13 respectively.

A second dielectric insulating (mica disc) member 8 is then positioned at the top of the assembly of elements and rigidly united to lead wires 12 and 13 in an analogous manner to that employed with the first disc, openings being'provided therein for an extension of the grid orcontrol electrode to extend 'therethrough and for "the filament guide member 5 to extend therethrough. A guide sleeve member 19 consisting of a refractory metal eyelet with the sleeve thereof positioned with its longitudinal axis concentric with the axis of tubular sleeve member 2 is provided in the dielectric shield member 8 through which the filament guide member 5 extends, so that the said member may be rigidly united by are or spot welding the guide member thereto to assist in the positioning and aligning of the twin'coil helical filament 1. It is preferable that the two dielectric shield members 6 and 8 should compressively engage end plug members 4, 4 of the cathode so as to assist in the positioning of the cathode in the assembly and that means be provided so that lateral displacement of the elements as a result of shock and vibration be reduced to a minimum.

This method of assembly of the cathode and cooperating electrodes in this type tube in addition to eliminating deleterious electrical discharges heretofore causing undesirable hum noises, also materially reduces losses in manufacture due to assembly defects. The structure is substantially unified and the elements once assembled retain their mounted positions and spacings throughout subsequent manufacturing operations and operating life, and moreover the assembly is substantially free from deleterious microphonic noises caused by mechanical vibrations in loosely held parts.

As an illustration of the extent of the material advantage obtained by the use, of this type cathode in the 227 type cathode suchyas is disclosed in copending applications Serial; Nos.; 292',116'-and 372,33l above identified, a radio tube constructed in accordance-with the praeticeof the copending application has an average. minimum hum-value of about 2.0 millivoits. Bythe use of the present type cathode and heater element, maintaining all other factors constant, -the,hum characteris tics have been reduced to. less than 6 of a millivolt, which is'substantially negligible, and only measurable by an exceedingly sensitive hum tester employing an oscillograph. .v The :heating ,up time of the-cathode maybe varied somewhat depending upon the desired op erating temperature of the filament; Dueto the fact that thetwin-coil helical heater element is insulated: the heat absorption of the-insulating material must be compensated for bya sli htly higher, operating temperaturetinthe filament. In order to effect ,anincrease in the operating temperature of the filamenta smaller diameter filament may beemployed. Areiatively slight decrease in filament diameter very eife'ctively changes the heatingup time of the cathode by raising the operating temperature or" the filament a. marked degree. The operating temperature'of the filament; however, must be maintained below the temperature of active electron emission therefrom in accordance with the invention of copendin'g application {372,331 above identified.) In electrondischarge devices of. the-224 or screen grid type employing indirectly heated cathodes, the replacement ofthe heretofore em rployed insulated hairpin type heater element by the. twincoil type insulated helical heater element of the present invention does notsubstantially effect a material alteration in the spacing of elements in the cathode structure heretofore employed .or in the spacedizrelationshipoof the electrodes thereto; q l I i In Fig; is shown a side elevational viewof this type of tube with the electrode. structure partially in crossrsection to illustrate the spaced relation? ship of said elements. In this type tube'the cathe ode sleeve member of the cathode instead of being 126 mil to 140 mil. diameter in the 227- type tube is approximately 64"mil to not o.ver:90 mii, de-- pending upon the. particular tube characteristics desired. It is desirable to employ in each a heater element 7 having substantially identical current consuming properties. It. is also desirable 1-,for manufacturing economy to employ in each type tube heater elements which are substantially r identical insize shape, diameter, length, etch Heretofore this :type tube hasfemployed indirectlyheated cathodes of the old type wherein there is r interposed between the cylindrical cathw ode sleeve member and. a hairpin; shaped heater element a refractory insulating core,'the heater element being threaded through openings traversing the longitudinal axis; of the core.

i The prolongedheating up time, the required high operating temperature of the heater element,

and; the. difiiculty ofleliminating by dielectric i shields the. deleterious-1 electric discharges 1 char-E aeterizedv by "fhum sounds in the device during operating the same were disadvantages which are-entirelymovercome by theinsertion in said cathode sleeve member 2 of the present type noninductive twin; wound insulated helicalc-athode heater element, intheplace of the old type heater. Fig. 5 may be seen heaterelernent 1 in posi:- tion in sleeve. member with control grid 20, screen grid 21, and x-plate l electrode 22 concentrically aligned thereto.=mBy the insertion of the twin. coili heater element. of the present, invention in the cathode.-of;1the 22'gtype screen grid tube the'heating up, time of, the cathode has been lowered' from approximately 140 seconds as with vold type heaterlto between-fi and 6 seconds, and at the same time without"aiteringthe regular assembly t'of' elements. -:'h eretofor,e employed, the fhum 'characteristic has beenreduced from about l o millivolts to notover'Lo millivolt, under standard test conditions. n v

It is apparent from the above description and the attacheddrawingzthat there maybe many applicaticns and. adaptations made of the; specific type of non-inductive twinwound insulated helical heater element in electron discharge devices enriployingv ahot cathodeof the indirectly heated type-without essentially departmge from the na-- ture-and scope of the presentinvention as set forth in the 'followingclaim I i What is'claimedisLr t- An electron discharge device comprising an envelope ha ving aipress,fa -circular mica disc spaced from said DI'BSS,..& circular mica disc spaced from saidw-firstdisc, two support rods sealed in said press} said discs having openings therein through which said rods project, eyelets in said openingsfor' positively engaging said rods 7 and'resting on said discsyanan'ode, cathode and grid located in the spac'ebetween said discs, ,said

grid being connected tozone of said supporting rods, andsaid' anode beingconnected to the other of saidsupportingrods. f

p WARLE Y'LnPARRO' 

